This invention relates generally to a web feeding apparatus, and more particularly to an apparatus for applying variable web tension to the web for effecting web registry between web passes through a web processing machine.
Arrangements are known for the feeding of the web receiving multiple operations which are required to be in registration. A typical example is multi-color printing, or scoring, creasing, punching and cutting of the web already printed, thus requiring a "print-to-print" register or a "cut-to-print" register. Between the first processing operation of the web, such as printing, and a subsequent processing operation, the web is normally wound into a roll and thereafter unwound as it is again fed through the web processing machine. Care must be taken in maintaining registration between web passes so that the subsequent printing operation, for example, does not interfere with the print already applied to the web during its first pass through the machine. It becomes particularly difficult to maintain such registration for webs of paper, foil, fabric and the like which stretch to various extents therealong depending on the moisture content. Also, during production, the webs may be rendered non-homogenous thereby causing the web to yield to variable extents when subjected to a uniform tension. Moreover, repeat lengths applied along the web must typically be precisely controlled relative to each other.
U.S. Pat. No. 3,025,791 discloses a technique for effecting variable web tension as the web is scanned during its passage through a multi-color press, as well as detecting the phase of the plate cylinders of the printing machines. An adjustable dancer roll is provided for applying a maintained tension to the web between a web pull unit and the first of the web processing machines. A controllable weight element is provided for varying the adjustment of the dancer roll, and control circuitry responsive to a device which scans markings applied to the web operates the weight element for varying the tension applied to the web ahead of the machines. However, such an arrangement is not readily conducive for controlling registration of a previously printed web temporarily rewound and again fed through the printer. Moreover, such an arrangement does not make provision for a clear separation of the total stretch of the web as well as for variable stretches along the length of the web.
In French Pat. No. 1,525,118 a pair of dancer rolls for applying tension to the web is disclosed as located respectively between the print couple and the web draw mechanism and between the draw mechanism and the web supply roll. The latter dancer roll is, however, utilized for braking the unwound web, while the other dancer roll is provided for controlling the r.p.m. of the draw mechanism. Web tension as it passes through the machine is, therefore, influenced by the control of both dancer rolls except that no provision is made for controlling both the total web tension as well as changes in web tension along the web. Hence, such an arrangement is not readily adaptable for registering a preceding web processing operation with a succeeding web processing operation.
In German Pat. No. 563,554 both a dancer roll as well as an adjustable guide roll are disclosed for controlling the tension of the web fed through a web processing machine. However, existing markings on the web are not scanned and compared during a succeeding web processing operation. Moreover, since the rolls are not interrelated, this arrangement is not suitable for use in processing thin webs, especially those running at high speeds, since webs are apt to tear easily. Besides, this arrangement includes multiple parts cooperating mechanically and coupled together thereby affecting the precision thereof.
In German Pat. No. 615,035, an arrangement is disclosed wherein registration is controlled by the scanning of the markings on the web to be processed and, if deviations from a set value of web stretch occur, corresponding corrections in web tension are made by changing the speed of the draw mechanism and by simultaneously adjusting a tension roll. By controlling the speed of the draw mechanism, web tension is altered during its subsequent passage through the web processing equipment, except that the precision with which the arrangement operates is insufficient for present day requirements since no provision is made for controlling variable web tension.